If you don't know, now you know: POPOS are plazas, atriums, and small parks that are part of downtown San Francisco office buildings. Maybe you've walked by them, maybe you've explored them: They're "Privately Owned Public Open Spaces," and while until 1985 they were often built voluntarily or in exchange for density bonuses, since that year, which saw the SF Downtown Plan, they've been mandatory.
However, mandating they be publicized hasn't been as easy as mandating they be built. These spaces often exist as open secrets, quiet places of repose known only to a few beyond the office workers who occupy the building surrounding them.
In fact, it wasn't until 2011 that the Planning Department began to keep a database of SF POPOS — there are nearly 70 now — and it wasn't until 2012 that the city approved legislation to enforce plaques and signage to indicate POPOS to the public. I first checked out POPOS as part of Rick Evans' San Francisco Architecture walking tour, a worthy investment for any enthusiast.
Today, the Chronicle indicates a new and somewhat-exciting POPOS in the Linkedin building at 222 2nd Street. That actually opened in March, and it's enticing in part because it's got restrooms, some cafe space, and excellent Frank Stella artwork.
While the Chronicle's architecture critic basically panned the building when it was completed, the public space caught a bit of a break. In a review where John King called the building an "alien presence" and a "black hole," he did note the "undeniably generous public room," though he figured it better for a photo shoot or a walk through than a sit and a cup of coffee. The public will be the judge of that, so go see for yourself.